Neuronal death after initial insult to central nervous system (CNS) tissue leads to a vicious cycle of neurotoxicity that, unless homeostasis is rapidly restored, results in spread of damage (Doble, 1999). In case that the evoked mechanisms of protection and repair are insufficient, further death of neurons will occur, a phenomenon relevant to many CNS pathologies regardless of their etiology or primary cause of damage including the nerve cells of the retina and the optic nerve, which are part of the CNS. In all deviation from homeostasis resident immune cells are activated. In the CNS such immune cells are the resident microglia. If these cells are not sufficiently activated, or activated but are not resolved on time, they may enter into a vicious cycle of local inflammation, which is not the primary cause of the pathology but may be part of its progression. Halting insufficient local immune activity of an immune activity that is not arrested on time requires the help of inflammation-resolving cells, circulating monocytes that are freshly recruited and can be locally educated to become inflammation-resolving cells. Below we will summarize eye diseases in which immune cells have been implicated.
Age-related macular degeneration. (AMD) is a disease affecting the macular region of the eye, which is the area in the retina where the sharp vision is obtained, manifested by atrophy of the retinal pigment epithelial layer below the retina and loss of photoreceptors (rods and cones). About 10% of AMD patients suffer from neovascular or exudative macular degeneration caused by the deterioration of the central portion of the retina due to abnormal blood vessel growth. Diverse cellular processes have been implicated in AMD pathogenesis, including inflammation, oxidative stress, altered cholesterol metabolism and/or impaired function of the retinal pigment epithelium. The proliferation of abnormal blood vessels in the retina is stimulated by vascular endothelial growth factor (VEGF).
Uveitis is thought to be caused by autoimmune disorders such as rheumatoid arthritis or ankylosing spondylitis, infection, or exposure to toxins. However, in many cases the cause is unknown. The most common form of uveitis is anterior uveitis, which involves inflammation in the front part of the eye. The inflammation may be associated with autoimmune diseases, but most cases occur in healthy people. Posterior uveitis is a potentially blinding ocular inflammatory condition that affects the choroid of the eye and the neural retina. Many studies have identified macrophages as key players in experimental autoimmune uveitis (EAU), a model for human posterior uveitis, mainly in the context of the induction and effector phase of the disease, although several reports have indicated the presence of macrophages in the resolution stage, as well. However, in most of these studies there was no functional discrimination between resident microglia and infiltrating macrophages. In addition, the characterization of distinct subsets of monocyte-derived macrophages and their functions at different phases of the disease in vivo have not yet been reported.
Retinitis pigmentosa (RP), a form of retinal dystrophy, is a group of inherited disorders characterized by progressive peripheral vision loss and night vision difficulties (nyctalopia) that can lead to central vision loss. RP is caused by abnormalities of the photoreceptors (rods and cones) or the retinal pigment epithelium (RPE) of the retina leading to progressive sight loss. Clinical evidence of sustained chronic inflammatory reaction in RP was shown, suggesting that inflammation may underlie the pathogenesis of RP (Yoshida et al., 2013).
Diabetic retinopathy, is retinopathy caused by complications of diabetes, which can eventually lead to blindness. Diabetes causes a number of metabolic and physiologic abnormalities in the retina, but which of these abnormalities contribute to recognized features of diabetic retinopathy (DR) is less clear. Many of the molecular and physiologic abnormalities that have been found to develop in the retina in diabetes are consistent with inflammation. Moreover, a number of anti-inflammatory therapies have been found to significantly inhibit development of different aspects of DR in animal models (Tang et al., 2011).
Glaucoma is a slow-progressing optic neuropathy with a high incidence in the elderly population (approximately 1%). Until recently, it was associated with high intraocular pressure (TOP) and therefore attempts have been focused on slowing down the disease progression by anti-hypertensive drugs. Over the years, it became apparent that glaucoma is a family of diseases and not all are associated with pressure. Moreover, it became clear that even when the disease is associated with pressure, the latter may be reduced to normal and even below normal values and degeneration may continue. An ongoing discussion among clinicians has questioned whether the continuous degeneration in glaucomatous patients, in spite of normal IOP values, is a reflection of the existence of additional risk factors besides pressure or a reflection of the increased vulnerability of the remaining neurons and fibers and thus the need to reduce IOP below normal values. Glaucoma results in neurodegeneration especially of the retinal ganglion cells (RGCs) leading to progressive loss of visual field up to loss of vision.
Etiology. Acute and/or chronic neuronal loss in the adult CNS results in the irreversible loss of function due to the very poor ability of mature nerve cells to proliferate and compensate for the lost neurons. Thus attenuating or reducing neuronal loss is essential for preservation of function. In most of the neurodegenerative diseases the etiology is not clear, hence they are incurable. Nevertheless, there are some primary and secondary risk factors, which are the target for therapeutic intervention aiming at inhibiting or attenuating progress of neuronal loss, collectively termed as neuroprotective therapy. Some of the risk factors are disease-specific but others, like excitatory amino acids, free radicals and nitric oxide, are common to all the neurodegenerative disorders. These factors are essential self-components in the healthy CNS, but with their accumulation in excess amounts in the degenerative tissue, they become cytotoxic leading to the spread of damage beyond the initial cause of neuron death.
Glutamate is one of the most common mediators of toxicity in acute and chronic degenerative disorders (Pitt et al., 2000). Glutamate is a primary excitatory neurotransmitter in the human CNS. L-glutamate is present at a majority of synapses and is capable of displaying dual activity: it plays a pivotal role in normal functioning as an essential neurotransmitter, but becomes toxic when its physiological levels are exceeded.
Current disease management. There is currently no cure for neurodegenerative diseases, and treatment focuses on alleviating or managing symptoms. The neuroprotective therapy has failed to show efficacy in the vast majority of the clinical studies that were conducted so far. Regarding AMD, anti-angiogenics or anti-VEGF agents can cause regression of the abnormal blood vessels and improve vision when injected directly into the vitreous humor of the eye
Regenerative medicine and protective autoimmunity Regenerative medicine is an emerging field that aims to repair, replace, and/or regenerate damaged tissues and organs by stimulating previously irreparable organs into healing themselves including tissue engineering, biomaterials, and cellular therapy. Cell renewal, a common healing process in peripheral tissues, is limited in the adult neural retina as it is in the rest of the CNS. However, a quiescent population of retinal progenitor cells (RPCs) continues to exist in the retinal ciliary body (CB) throughout adulthood and has the potential to differentiate into various cells of the retina or to possibly serve as a source of immunomodulatory or neurotropic agents. This dormant progenitor cell niche was reported to be stimulated after retinal injury, although the underlying mechanisms are yet to be revealed. Unraveling the healing processes that operate in response to injury and finding ways to enhance them could lead to the development of new therapies for promoting neuroprotection and cell renewal, which is among the research goals in this field.
Outside the CNS, healing processes require the help of the immune system for clearance of dead cells and cell debris and for support of regrowth and cell renewal. These processes are mediated, in part, by different subsets of macrophages that acquire discrete phenotypes over the time course of healing. In the course of a response to any insult, there is a pivotal stage of termination of the local immune response involving monocyte-derived macrophages, which contribute to an overall anti-inflammatory milieu and produce growth factors needed for regeneration.
Infiltration/recruitment of peripheral blood into the lesion site is controlled by signals elicited from the lesion site, which affects the brain-cerebrospinal fluid (CSF) barrier. The limited spontaneous recovery following CNS injury can be attributed in part to the inadequate, untimely, spontaneous recruitment of the effective subset of monocytes to the lesion site. The above considerations have led the inventors to follow a novel physiological approach that employs the body's professional healing system, the immune system, to contend with the consequences of CNS damage leading to neuroprotection and restoration.
It has been shown in the laboratory of the inventors that blood-derived monocytes incubated with skin segments acquired a non-inflammatory phenotype similar to anti-inflammatory M2 monocytes described in the literature. Injection of the monocytes into the injured spinal cord induced better recovery from spinal cord injury (SCI) in rats (U.S. Pat. Nos. 5,800,812; 6,117,424; and 6,267,955). This approach was tested in a clinical study on patients suffering from acute sever spinal cord injury showing encouraging results (WO 03/044037; Knoller et al., 2005). The inventors have also shown that enrichment of peripheral blood monocytic pool with bone-marrow derived CD115+ cells augmented functional recovery following SCI in mice (Shechter et al., 2009).
Blood monocytes are heterogenic cellular population with different characteristics and activities. Utilizing blood monocytes for therapeutic purposes requires the identification of the cells with harmful functions and those that are beneficial.
In humans, it was proposed that the expression of CD16 on monocytes can distinguish between three subsets, namely CD14++CD16− (classical) CD14++CD16++ (intermediate) and CD14dimCD16++ (non-classical) monocytes, but their role in physiological and pathological conditions is not fully understood. Recently, the inventors discovered that CD16+ enriched monocytes, upon injection into the CSF of animals following spinal cord injury, attenuates spontaneous recovery as compared with animals that received total monocytic sub-population. It was also found that a monocytic sub-population devoid of CD16 expressing cells (CD16−), are beneficial for recovery following spinal cord injury (PCT/IL2012/050522).